Modeling cosmogenic radionuclides

All existing 10 Be records from Greenland and Antarctica show increasing concentrations during the Maunder Minimum period (MM), 1645–1715, when solar activity was very low and the climate was colder (little ice age). In detail, however, the 10 Be records deviate from each other. We investigate to what extent climatic changes influence the 5 10 Be measured in ice by modeling this period using the ECHAM5-HAM general circulation model. Production calculations show that during the MM the mean global 10 Be production was higher by 32% than at present due to lower solar activity. Our modeling shows that the zonally averaged modeled 10 Be deposition flux deviates by only ∼8% from the average increase of 32%, indicating that climatic effects are much smaller than 10 the production change. Due to increased stratospheric production, the 10 Be content in the downward fluxes is larger during MM, leading to larger 10 Be deposition fluxes in the subtropics, where stratosphere-troposphere exchange (STE) is strongest. In polar regions the effect is small. In Greenland the deposition change depends on latitude and altitude. In Antarctica the change is larger in the east than in the west. We use the 15 10 Be/ 7 Be ratio to study changes in STE. We find larger change between 20 • N–40 • N during spring, pointing to a stronger STE in the Northern Hemisphere during MM. In the Southern Hemisphere the change is small. These findings indicate that climate changes do influence the 10 Be deposition fluxes, but not enough to significantly disturb the production signal. Climate-induced changes remain small, especially in polar 20 regions.